Healthy Head?

Looks and sounds like the head wants enough work that you'll want it properly rebuilt, including guides and hard exhaust seats. Careful-careful; putting a nice, good-sealing head atop an old bottom end is a sure-fire recipe for an oil burner de luxe (pulls oil past worn rings/bore), and this engine was already beginning to smoke, so you'd be smart to refurbish the whole engine. This will involve the usual thorough cleaning (inside and out—there's probably quite a bit of rust and mud in the coolant passages) and extensive careful measurement to determine what-all is needed to bring it up to snuff.

There is a great deal of low-hanging fruit in the form of parts and assembly tricks and techniques to wind up with a better result. For example, the camshaft and its bearings are wear items, so take the opportunity to get a № 2106R camshaft from Oregon Cam—that's the Dutra RV-10RDP, which brings a really nice improvement in driveability on a street-driven car without the drawbacks and hassles of most of the more widely advertised "upgrade" cams for the Slant-6. You'll find plenty of build and parts advice here and on slantsix.org.